(1) Field of the Invention
This invention relates to a gray-scale reference voltage driving circuit for liquid crystal displays (LCDs), and more particularly, to a reference voltage driving circuit with a compensating circuit for calibrating reference voltage.
(2) Description of Related Art
Active matrix liquid crystal displays (AMLCDs) apply various electric fields to control the transparency of liquid crystal molecules for displaying images. As shown in FIG. 1A, a traditional AMLCD 1 includes a display panel 10 and a driving system 20. The display panel 10 has a pixel array 12. Each pixel device 120 of the pixel array 12 includes a liquid crystal capacitor 122 and a thin film transistor (TFT) 124. The liquid crystal capacitor 122 is composed of a pixel electrode, a common electrode, and a liquid crystal layer. The potential difference between the pixel electrode and the common electrode decides the transparency of the liquid crystal layer. The TFT 124 is electrically connected to the pixel electrode and utilized as a switch for selectably charging the pixel electrode.
The driving system 20 has a control circuit 22, a source driver 24, and a scanning driver 26. The control circuit 22 is utilized for applying digital displaying data DD and control signal CS to the source driver 24, and also applying scanning signal SS to the scanning driver 26. Also referring to FIG. 1B, the source driver 24 has a latch circuit 246, a plurality of digital to analog converters (DACs) 242, and a reference voltage driving circuit 244. The latch circuit 246 is utilized for latching the digital displaying data DD according to the scanning sequence defined by the control signal CS. The DACs 242 are utilized for transforming the latched displaying data into source driving voltage Vs. The source driving voltage Vs is then applied to the TFTs 124 of the pixel array 12 column by column through the data lines 32. The scan driver 26 is utilized for transforming the scanning signal SS into gate driving voltage Vg and applying the gate driving voltage Vg to turn on the TFTs 124 of the pixel array 12 row by row through the scanning lines 34.
Since the levels of the source driving voltage Vs decide the brightness of the pixel devices 120, in order to reduce the loss in the transformation between the digital display data DD and the display signals, the reference voltage driving circuit 244 within the source driver 24 must have accurate reference voltage values with respect to different gray-scale displaying brightness provided. Thus, the source driver 24 can transform the digital displaying data DD into the ideal source driving voltage Vs according to the reference voltage provided by the reference voltage driving circuit 244. On the contrary, as the mismatch of the reference voltage values provided by the reference voltage driving circuit 244 with respect to the ideal gamma curve exists, the accurate displaying images on the display panel 10 are not available.
Referring to FIG. 2, the reference voltage driving circuit (labeled 244 in FIG. 1B) has a resistor string composed of a plurality of resistors R0, R1, R2 . . . Rk connected in a serial. The resistor string has a grounded end GND and a power supply end applied with a potential VCC. A plurality of output nodes P0, P1, P2 . . . Pk is located between every neighboring resistors R0, R1, R2 . . . Rk for providing reference voltages with various levels relative to different gray scales. Also referring to FIG. 1B, the reference voltages from the output nodes P0, P1, P2 . . . Pk are applied to the DACs 242 directly. In addition, each DAC 242 electrically connected to output nodes P0, P1, P2 . . . Pk of the resistor string results to a significant error of the reference voltages from the output nodes P0, P1, P2 . . . Pk.
In order to minimize the bad influence from the DACs 242, a plurality of negative feedback operation amplifiers OA0, OA1, OA2 . . . OAk are electrically connected between the output nodes P0, P1, P2 . . . Pk and the respecting DACs as a buffer.
However, the additional operation amplifiers OA0, OA1, OA2 . . . OAk connecting to the resistor string may shift the potentials of the reference voltage provided from the output nodes P0, P1, P2 . . . Pk. The reference voltage levels applied to the DACs 242 in reality, which are provided from the output end of the operation amplifiers OA0, OA1, OA2 . . . OAk, are different from the preset voltage levels provided from the output nodes P0, P1, P2 . . . Pk of the resistor string. For example, as shown in FIG. 3, assuming the operation amplifier OAi has a gain g and the reference voltage value provided from the resistor string is Vin, the reference voltage value Vout provided from the output end of the operation amplifier OAi equals to Vin(g/(1+g)) and a reference voltage error of Vin(1/(1+g)) is resulted because of the connected operation amplifier OAi.
Generally speaking, the reference voltage error can be reduced through increasing the gain g of the operation amplifier OAi. The gain g of the operation amplifier OAi is decided by the difference between the two potentials VDD and VSS applied thereto. However, the increasing of the difference between the two potentials VDD and VSS usually needs a higher voltage input and also an increased power consumption of the liquid crystal display panel.
In addition, attending with the development of the advance low temperature polysilicon (LTPS) fabrication technology, such as laser crystallization, forming polysilicon thin film transistors on the displaying panel to result the system on glass (SOG) liquid crystal display with reduced size and weight has become an important subject. However, the LTPS fabrication technology nowadays has a major drawback of poor uniformity, which usually leads to some uncertainty of the operation amplifiers formed on the glass substrate. Thus, as the reference voltage driving circuit 244 of FIG. 2 is formed on a glass substrate in such a manner, the reference voltage errors become unpredictable.
Accordingly, how to reduce the errors resulted by the negative feedback operation amplifiers connected with the resistor string and how to overcome the poor uniformity of the operation amplifiers formed by the LTPS fabrication technology have become important issues for circuit design in LCD industry.